1. Technical Field
This disclosure relates to a heat-sensitive recording material used in the field of printers for such as computer outputs and calculators, recorders for medical measurements, low-speed and high-speed facsimiles, automatic ticket machines, and handy terminals.
2. Description of the Related Art
So far, various heat-sensitive recording materials have been proposed that have on a substrate a heat-sensitive color developing layer containing at least a leuco dye and a developer and utilize a color developing reaction between the leuco dye and the developer. Such heat-sensitive recording materials have advantages including (1) no need for subjecting them to a cumbersome treatment such as development and fixation and ability to record in a short time using a relatively simple apparatus, (2) less noise production, and (3) less costs. Such a heat-sensitive recording material is thus used for copying books and documents, for example, and used widely as a recording material for electron computers, facsimiles, ticketing devices, label printers, recorders, and handy terminals.
Heat-sensitive recording materials are desired to develop colors of high density promptly and to develop highly tough color images and backgrounds. In recent years, heat-sensitive recording materials are increasingly frequently used also in fields where reliability of recorded images is regarded as important, such as in labels, and are more and more desired to have high storage stability against plasticizers and oils and fats in organic polymer materials used in packages of the heat-sensitive recording material.
Furthermore in recent years, in order to conserve energy for a better environment or to make recording materials compatible with portable thermal printers, a technology is studied that is able to develop colors with a low printing energy input. Also in order to make recording materials compatible with high-speed printing using a high-speed printer, a technology to supersensitize a heat-sensitive paper is studied. However, since a heat-sensitive recording material is a compound to develop colors by heat, intensity of colors developed is enhanced depending on the heat applied. Thus, when a highly sensitive heat-sensitive recording material is left in summer in a room where the temperature is high, or when it is stuck as a heat-sensitive recording label and heated by a microwave oven, there has been a disadvantage that the background becomes black and it becomes difficult to read characters and bar-code (this phenomenon is called as ‘background fog’). Thus for heat-sensitive recording materials, it is challenged to balance color developing sensitivity and heat resistance as much as possible.
As related arts, heat-sensitive recording materials using a 4-hydroxyphenyl-4′-alkoxyphenyl sulfone derivative are disclosed in Japanese Patent Application Laid-Open (JP-A) No. 62-225391 and Japanese Patent Application Publication (JP-B) No. 63-61198. Specific examples include 4-hydroxyphenyl-4′-octyloxyphenyl sulfone, 4-hydroxyphenyl-4′-dodecyloxyphenyl sulfone, 4-hydroxyphenyl-4′-benzyloxyphenyl sulfone, 4-hydroxyphenyl-4′-isopropoxyphenyl sulfone.
However, heat-sensitive recording materials in which these 4-hydroxyphenyl-4′-alkoxyphenyl sulfone derivatives are simply used in combination with leuco dyes, have disadvantages that they have insufficient color development and heat resistance storage stability at the image portion and the background portion.
It is one of technically widely used methods to combine these developers and a leuco dye of a high melting point and to add further a sensitizing agent to enhance heat resistance without reducing color developing sensitivity. However, a heat-sensitive recording material where these 4-hydroxyphenyl-4′-alkoxyphenyl sulfone derivatives and a leuco dye of a high melting point are combined and a sensitizing agent is further added, frequently exhibits other disadvantages. Therefore an effective sensitizing agent was not available so far. Examples of the disadvantages include background fog and discoloration of the image portion with time, which were limiting the usage of above heat-sensitive recording material using a combination of 4-hydroxyphenyl-4′-alkoxyphenyl sulfone derivatives and a leuco dye with addition of a sensitizing agent.
Also as related arts, a heat-sensitive recording material using 4-acetylbiphenyl is disclosed in JP-A No. 61-246088 and JP-A No. 10-138645, and the like. However, when 4-acetylbiphenyl is used as a sensitizing agent, a simple mere combination of 4-acetylbiphenyl and developers resulted in insufficient heat resistance storage stability of the image portion and the background portion at 80° C.